The invention refers to a surface acoustic wave arrangement wherein a piezo substrate is provided with a plurality of interdigitated metallic fingers on a surface thereof forming input, output, and coupling transducers.
From the state of the art, in particular television device circuits, surface acoustic wave filters are known wherein at least one transducer, for example an input transducer has an interdigitated structure formed of meshing comb-like conductive transducer fingers on a substrate with piezoelectric effect. The transducer conductive fingers have different lengths and adjacent transducer fingers have overlapping which varies. With these differing finger lengths and differing overlapping, the transmission function of the transducer can be pre-set in a way that is necessary for the particular use desired. This technique of providing overlapping is designated as a weighting of the transducer fingers.
The different lengths of the transducer fingers result in an active portion of the transducer which does not necessarily have a width which remains the same over the entire length of the transducer. The length is defined as being parallel and the width is defined as being transverse to the propagation of mechanical piezoelectric induced surface acoustic waves. Surface acoustic waves which are produced in the initial part of the transducer and from there proceed outward, would then, during their passage through the transducer, pass through surface areas of a substrate body as viewed over a width of the transducer where either none or an unequal number of transducer fingers are present. For a uniform, spurious-free path for the surface acoustic waves, in particular so as to avoid transit time differences in neighboring areas transverse to the wave propagation direction, one also uses transducer fingers which have significant portions of their length inactive in the transducer because of the lack of overlapping opposed polarity length portions of adjacent fingers in this area. Next to a given finger there is present no transducer finger of opposed polarity. Such portions are designated as dummy fingers.
In the case of the surface acoustic wave arrangements as described with an input transducer and an output transducer for transmission of a signal from the input to the output, signals appear which are spurious and which have transit time shifts opposed to the signal transmission. For example, such a spurious signal can exist with a portion of the surface acoustic waves which have already arrived in the output transducer in the area of the transducer fingers of the output transducer. There they experience a mechanical and/or electrical reflection. The wave passes back through the surface acoustic wave arrangement to the input transducer as a reflected signal in a direction opposed to the intended direction, and there again, in part is reflected back mechanically and/or electrically. Such a signal which passes through the surface acoustic wave arrangement three times is designated as a triple transit signal.